DE69211943T2 - Rubber mixtures which contain an oligomeric maleimide and / or a maleic acid - Google Patents

Description

Background of the invention

A pneumatic tire is a polymeric composite and is a complex system of interacting components, each with specific properties for maximum effectiveness. A common problem in manufacturing a rubber composite is maintaining good adhesion between the rubber and the reinforcement. A conventional method in promoting adhesion between the rubber and the reinforcement is to pretreat the reinforcing fiber with a mixture of a rubber latex and a phenol-formaldehyde condensation product, in which the phenol is almost always resorcinol. This is the so-called "RFL" (resorcinol-formaldehyde latex) process. An alternative method of promoting such adhesion is to generate the resin in situ (in the vulcanized rubber/fabric matrix) by compounding a vulcanizing rubber bulk composition with the phenol-formaldehyde condensation product (hereinafter referred to as the "in situ method"). The components of the condensation product consist of a methylene acceptor and a methylene donor. The most common methylene donors include N-(substituted oxymethyl)melamine, hexamethylenetetramine, or hexamethoxymethylmelamine. A common methylene acceptor is a dihydroxybenzene compound such as resorcinol. The in situ method has been found to be particularly effective in cases where the reinforcing material is steel wire, since it has been observed that pretreatment of the wire with the RFL system is largely ineffective.

Resorcinol is known to form a resin network within a rubbery polymer by reacting with various methylene donors. Unfortunately, the use of resorcinol has some inherent disadvantages. Resorcinol is not readily dispersed in rubber, and in fact neither the resin nor the resorcinol are chemically bonded to the rubber. In addition, resorcinol in its raw form is extremely volatile and is potentially toxic, thus posing a health hazard. Another disadvantage of using resorcinol is periodic supply shortages on the market.

There have been numerous attempts to replace resorcinol, but few, if any, have had much success. For example, U.S. Patent 4,605,695 discloses a method for improving the adhesion of rubber to reinforcing materials by using phenolic esters as a methylene acceptor. These phenolic esters are less volatile than resorcinol, but still do not provide a readily reactive site for chemical attachment of the resin to the rubber.

Therefore, there is a need to find a suitable adhesion promoter.

Summary of the invention

The present invention relates to a sulfur-vulcanized rubber compound comprising a sulfur-vulcanized rubber and 0.1 to 10.0 phr of an oligomeric maleimide of the formula:

or a maleamic acid of the formula:

wherein R and R¹ are individually selected from the group of radicals consisting of hydrogen, an alkyl having 1 to 4 carbon atoms or a halogen; R² is selected from the group of radicals consisting of 1 to 12 carbon atoms; X has a value of 1 to 146 and n has a value of 0 to 4.

Detailed description of the preferred embodiment

The maleimide oligomer or maleamic acid can be used in various concentrations in the rubber compounds of the present invention. For example, the concentration can range from about 0.1 to 10.0 parts by weight per 100 parts of rubber (also known as "phr"). Preferably, the concentration ranges from about 0.5 to about 5.0 phr.

In accordance with the preferred embodiment, R is hydrogen, R¹ is hydrogen, R² is an alkyl having 1 carbon atom and n has a value of 0.

The maleamic acids used in the present invention can be prepared by condensing certain diamines with maleic anhydride or substituted maleic anhydride. Representative diamines that can be used include α,α'-bis-(4-aminophenyl)-meta-diisopropylbenzene and α,α-bis-(4-aminophenyl)-para-diisopropylbenzene.

Maleic anhydride or substituted maleic anhydride is reacted with the above diamine compound under suitable conditions to form the maleamic acid. The anhydride can be reacted with any of the above diamine compounds in a variety of molar ratios. Generally, the molar ratio of the anhydride to the diamine compound ranges from about 2.5:1 to about 0.75:1, with a range of about 2.1:1 to about 1.9:1 being preferred.

An organic solvent can be used to dissolve the anhydride or diamine compound. The solvent is preferably inert to the reaction between the anhydride and the diamine compound. Illustrative examples of solvents suitable for use in the practice of this invention include: saturated and aromatic hydrocarbons, e.g., hexane, octane, dodecane, naphtha, decalin, tetrahydronaphthalene, kerosene, mineral oil, cyclohexane, gydoheptane, alkylcycloalkane, benzene, toluene, xylene, alkylnaphthalene, and the like; acetone; Ethers such as tetrahydrofuran, tetrahydropyran, diethyl ether, 1,2- dimethoxybenzene, 1,2-diethoxybenzene, the mono- and dialkyl ethers of ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, oxyethyleneoxypropylene glycol and the like; fluorinated hydrocarbons which are inert under the reaction conditions such as perfluoroethane, monofluorobenzene and the like. Another class of solvents are sulfones such as dimethyl sulfone, diethyl sulfone, diphenol sulfone, sulfolane and the like. Mixtures of the above solvents can be used as long as they are compatible with each other under the reaction conditions and sufficiently dissolve the diamine or anhydride compound and do not interfere with the reaction.

The reaction between the anhydride and the diamine compound to form the maleamic acid can occur over a wide range of temperatures. The temperature can range from moderate to elevated. Generally, the reaction can be carried out at a temperature between about 20°C and 100°C. The preferred temperature range is about 30°C to 80°C, while the most preferred temperature range is about 55°C to 65°C.

The reaction to form maleamic acid can be carried out under a variety of pressures. Pressures ranging from about 0 psig to 100 psig can be used.

The oligomeric maleimide used in the present invention is also prepared by condensing the above diamines with maleic anhydride or substituted maleic anhydride.

As in the preparation of maleamic acids, the molar ratio of anhydride to diamine, the organic solvents and the reaction pressures can be the same as described above.

The reaction between the anhydride and the diamine compound to form the oligomeric maleimide is carried out in the presence of a catalyst. Examples of catalysts that may be used include acidic catalysts such as sulfuric acid, hydrochloric acid, and toluenesulfonic acid. The amount of catalyst that may be used will vary depending on the particular catalyst selected. For example, when an acidic catalyst is used, about 5% to about 10% by weight of the diamine is recommended.

The reaction to produce the oligomeric maleimide is normally carried out at higher temperatures than used in the production of the maleamic acid. Generally, the reaction can be carried out at a temperature between about 100 and about 200°C. The preferred temperature range is about 120 to 180°C, while the most preferred temperature range is about 140 to about 160°C.

The reaction to form the oligomeric maleimide or maleamic acid is carried out for a period of time sufficient to produce the desired product. In general, the reaction time can vary from minutes to several hours. If the more sluggish reaction conditions are chosen, then the reaction time will have to be extended until the desired product is produced. It is clear that the residence time of the reactants will be influenced by the reaction temperature, the concentration and choice of the catalyst, the total gas pressure, the partial pressure exerted by its components, the concentration and choice of the solvent and other factors. When To produce the oligomeric maleimide, the reaction is desirably carried out until one molar equivalent of water has been removed.

The process for producing the oligomeric maleimide or maleamic acid can be carried out in a batch, semi-batch or continuous manner. The reaction can be carried out in a single reaction zone or in a plurality of reaction zones, in series or in parallel. The reaction can be carried out intermittently or continuously in an elongated tubular zone or in a series of such zones. The material of construction of the equipment should be such that it is inert during the reaction. The equipment should also be able to withstand the reaction temperatures and pressures. The reaction zone can be equipped with internal and/or external heat exchangers to regulate temperature fluctuations. Preferably, a means of agitation is available to ensure a uniform reaction. Mixing induced by vibration, shaker, stirrer, rotation, oscillation, etc. are all illustrative examples of the types of agitation means contemplated for use in preparing the composition of the present invention. Such agitation means are available and well known to those skilled in the art.

The use of the oligomeric maleimide or maleamic acid improves the properties of "sulfur-vulcanized elastomers or rubbers." The term "sulfur-vulcanized elastomer or sulfur-vulcanized rubber" as used herein includes both vulcanized forms of natural rubber and all of its various raw and reclaimed forms, and various synthetic rubbers. Representative synthetic polymers include the homopolymerization products of butadiene and its homologues and derivatives, such as methylbutadiene, dimethylbutadiene and pentadiene, as well as copolymers such as those formed from butadiene or its homologues or derivatives with other unsaturated organic compounds. Among the latter are acetylenes, e.g. vinyl acetylene; olefins, e.g. isobutylene which copolymerises with isoprene to form butyl rubber; vinyl compounds, e.g. acrylic acid, acrylonitrile (which polymerises with butadiene to form NBR), methacrylic acid and styrene, the latter polymerising with butadiene to form SBR; and vinyl esters and various unsaturated aldehydes, ketones and ethers, e.g. acrolein, methyl isopropenyl ketone and vinyl ethyl ether. Also included are the various synthetic rubbers prepared by the homopolymerisation of isoprene and the copolymerisation of isoprene and other diolefins in various unsaturated organic compounds. Also included are the synthetic rubbers such as 1,4-cis-polybutadiene and 1,4-cis-polyisoprene and similar synthetic rubbers.

Specific examples of synthetic rubbers include neoprene (polychloroprene), polybutadiene (including trans- and cis-1,4-polybutadiene), polyisoprene (including cis-1,4-polyisoprene), butyl rubber, copolymers of 1,3-butadiene or isoprene with monomers such as styrene, acrylonitrile and methyl methacrylate, and ethylene/propylene terpolymers, also known as ethylene/propylene/diene monomer (EPDM), and especially ethylene/propylene/dicyclopentadiene terpolymers. The preferred synthetic rubbers for use in the present invention are polybutadiene, polyisobutylene, butadiene-styrene copolymers and cis-1,4-polyisoprene.

The vulcanizable rubber compositions of the present invention may contain a methylene donor. The term "methylene donor" is intended to mean a compound capable of reacting with the oligomeric maleimide or maleamic acid and generating the resin in situ. Examples of methylene donors suitable for use in the present invention include hexamethylenetetramine, hexaethoxymethylmelamine, hexamethoxymethylmelamine, lauryloxymethylpyridinium chloride, ethoxymethylpyridinium chloride, trioxanehexamethoxymethylmelamine, the hydroxyl groups of which may be esterified or partially esterified, and polymers of formaldehyde such as paraformaldehyde. In addition, the methylene donors may be N- substituted oxymethylmelamines of the general formula:

wherein X is alkyl of 1 to 8 carbon atoms; R, R¹, R², R³ and R⁴ are individually selected from hydrogen, alkyl of 1 to 8 carbon atoms, the group -CH₂OX or their condensation products. Specific methylene donors include hexakis(methoxymethyl)melamine, N,N',N"-trimethyl-N,N',N"-trimethylolmelamine, hexamethylolmelamine, N-methylolmelamine, N,N'-dimethylolmelamine, N,N',N"-tris(methoxymethyl)melamine and N,N',N"-tributyl-N,N',N"-trimethylolmelamine. The N-methylol derivatives of melamine are prepared by known methods.

The weight ratio of methylene donor to the oligomeric maleimide or maleamic acid can vary. Generally speaking, the weight ratio will range from about 1:10 to about 10:1. Preferably, the weight ratio will range from about 1:3 to 3:1.

Vulcanization of the rubber compound of the present invention is generally carried out at conventional temperatures ranging from about 100°C to 200°C. Preferably, vulcanization is carried out at temperatures ranging from about 110°C to 180°C. Any of the conventional vulcanization methods, such as heating in a press or vulcanizing mold, heating with superheated steam or hot air, or in a salt bath, may be used.

In addition to the oligomeric maleimide or maleamic acid, other rubber additives can also be incorporated into the rubber compound. The additives commonly used in rubber vulcanizates are, for example, carbon black, tackifier resins, Processing aids, antioxidants, antiozonants, stearic acid, activators, waxes, oils and peptizers. As is known to those skilled in the art, depending on the intended use of the rubber compound, certain of the above-mentioned additives are usually used in conventional amounts. Typical carbon black additions comprise about 20 to 100 parts by weight of diene rubber (phr), preferably 30 to 80 phr. Typical amounts of tackifier resins comprise about 1 to 5 phr. Typical amounts of antioxidants comprise 1 to about 10 phr. Typical amounts of antiozonants comprise 1 to about 10 phr. Typical amounts of stearic acid comprise 1 to about 2 phr. Typical amounts of zinc oxide comprise 2 to 5 phr. Typical amounts of waxes comprise 1 to 5 phr. Typical amounts of oils comprise 5 to 40 phr. Typical amounts of peptizers comprise 0.1 to 1 phr. The presence and relative amounts of the above additives are not an aspect of the present invention.

Vulcanization of the rubber compound is carried out in the presence of a sulfur vulcanizing agent. Examples of suitable sulfur vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide, or sulfur-olefin adducts. Preferably, the sulfur vulcanizing agent is elemental sulfur. As is known to those skilled in the art, sulfur vulcanizing agents are used in an amount ranging from about 0.5 to 8 phr, with a range of 1.0 to 2.25 being preferred.

Accelerators are conventionally used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In some cases, a single accelerator system may be used, i.e. a primary accelerator. Conventionally, a primary accelerator is used in amounts ranging from about 0.5 to 2.0 phr. In another case, to activate and improve the properties of the vulcanizate, combinations of two or more accelerators may be used, consisting of a primary accelerator, generally used in a large amount (0.5 to 2.0 phr), and a secondary accelerator, which generally used in smaller amounts (0.01 to 0.50 phr). Combinations of these accelerators are known to produce a synergistic effect on the final properties and are somewhat better than those produced by using either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce satisfactory cures at ordinary vulcanization temperatures. Suitable types of accelerators which may be used include amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and the xanthates. Preferably, the primary accelerator is a sulfenamide. If a secondary accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.

The rubber compounds containing the oligomeric maleimide or maleamic acid can be used in the manufacture of composite products including tires, transmission belts, conveyor belts, printing rollers, rubber shoe heels and soles, rubber wringers, automobile floor mats, truck mud flaps, ball mill liners and the like. Preferably, the rubber vulcanizates are used in carcass ply or cover layer compounds for tires.

The following examples are presented to illustrate, but not to limit, the present invention.

example 1 Preparation of oligomeric bismaleimide

A dry 1 L three-necked round-bottomed flask was fitted with a modified Dean-Stark separator, a cold water condenser and a thermometer. The flask was charged with 98 g (1.0 mol) of maleic anhydride dissolved in 400 mL of m-xylene, 10 g of toluenesulfonic acid and 172 g (0.5 mol) of EPON HPT vulcanizing agent 1061-M from Shell Oil Company. EPON HPT is a mixture of the para,para, ortho,para and ortho,ortho isomers of α,α'-bis-(4-aminophenyl)diisopropylbenzene. The flask was purged with nitrogen and sealed under a nitrogen balloon. The flask was then heated to reflux at approximately 145°C and held until a stoichiometric amount of water (18 mL, 1 mol) was removed via the Dean-Stark trap. The flask was allowed to cool to room temperature and the resulting product, oligomeric maleimide, was poured into a container and dried in a vacuum oven at 80°C for several hours to remove the excess m-xylene solvent. The reaction yielded 302 g of light gray-brown colored solid with a melting point of 151-178°C. GPC analysis shows high molecular weight oligomers ranging from MW 1500 to 74000.

Example 2 Production of maleamic acid

A 3-liter three-necked round-bottom flask was equipped with a thermocouple, an electric stirrer, and a nitrogen balloon. The flask was charged with 98 g (1.0 mol) of maleic anhydride dissolved in 400 mL of m-xylene. EPON HPT (172 g, 0.5 mol) was added slowly using a funnel while the mixture in the flask was vigorously stirred at 42°C. An additional 400 mL of m-xylene was used to flush the amine into the flask. The reaction showed an exotherm from 42°C to 55°C. The reaction was stirred and maintained at a temperature of 50-55°C for four hours, then cooled to a temperature of 10°C with stirring by immersing the flask in ice water. The resulting product was filtered with a suction filter and air dried to produce 272 g of yellow-brown colored product with a melting point of 191-198ºC.

Example 3 Physical testing

Table I below shows the base rubber compound used in this example. The rubber compound was prepared in a three-stage Banbury mix. Unless otherwise indicated, all parts and percents are by weight. The cure data and other physical data for each sample are listed in Table II. Table I

Cure properties were determined using a Monsanto vibrating rheometer operating at a temperature of 150ºC and at a frequency of 11 hertz. A description of vibrating rheometers can be found in the Vanderbilt Rubber Handbook, edited by Robert O. Ohm (Norwalk, Connecticut, R.T. Vanderbilt Company, Inc., 1990), pages 554-557. The use of this cure meter and standardized values read from the curve are specified in ASTM D-2084. A typical cure curve obtained on a vibrating rheometer is shown on page 555 of the 1990 edition of the Vanderbilt Rubber Handbook.

In such an oscillating rheometer, compounded rubber samples are subjected to an oscillating shearing action of constant amplitude. The torque of the oscillating disk embedded in the mass being tested, which is required to cause the rotor to oscillate at the vulcanization temperature, is measured. The values obtained using this vulcanization test are very significant since changes in the rubber or compounding recipe are very easily detected. It is obvious that it is usually advantageous to have a fast vulcanization rate.

Table II below presents cure properties determined from cure curves obtained for the two rubber formulations prepared. These properties include a torque minimum (Min. Torque), a torque maximum (Max. Torque), minutes up to 25% of torque increase (t25 min.), and minutes up to 90% of torque increase (t90 min.).

The peel adhesion test was conducted to determine the interfacial adhesion between the different rubber formulations that were prepared. The interfacial adhesion was determined by pulling one compound away from another at a right angle to the undrawn specimen, with the two ends at a 180º angle from each other using an Instron machine. The contact area was determined from the placement of a Mylar film between the compounds during vulcanization. A window in the Mylar allowed the two materials to come into contact during the test.

Adhesion to nylon and flexten was evaluated using the Tire Cord Adhesion Test (TCAT). Compound adhesion to tire wire was also evaluated using the TCAT test. Samples were prepared and tested according to the procedures described by D. W. Nicholson, D. I. Livingston and G. S. Fielding- Russel, Tire Science and Technology (1978) 6, 114; G. S. Fielding-Russel and D. I. Livingston, Rubber Chemistry and Technology (1980) 53, 950; and R. L. Rongone, D. W. Nicholson and R. E. Payne, U.S. Patent No. 4,095,465 (June 20, 1978).

Shore hardness was determined according to ASTM-1415. Table II

As shown in Table II, Samples 2 and 3 showed improved adhesion values over the control. With respect to Sample 2, the stress-strain, Rheovibron and Shore A hardness values were improved over the control.

Example 4 Physical testing

Table III below shows the base rubber compound used in this example. The rubber compound was prepared in a two-stage Banbury mix. Unless otherwise noted, all parts and percents are by weight. Cure data and other physical data for each sample are shown in Table IV. Table III Table IV

The use of maleamic acid in sample 2 improved the values for adhesion, Rheovibron and Shore A hardness.

Claims (7)

1. A sulfur-vulcanized rubber compound comprising a sulfur-vulcanized rubber, about 0.1 to about 10.0 phr of an oligomeric maleimide of the formula: or a maleamic acid of the formula: wherein R and R¹ are individually selected from the group of radicals consisting of hydrogen, an alkyl having 1 to 4 carbon atoms or one; R² is selected from the group of radicals consisting of 1 to 12 carbon atoms; X has a value of 1 to 146 and n has a value of 0 to 4. 2. Compound according to claim 1, characterized in that R is hydrogen, n is 0 and R² is an alkyl with 1 carbon atom. 3. Compound according to claim 1, characterized in that the oligomeric maleimide is used. 4. Compound according to claim 1, characterized in that maleamic acid is used. 5. A compound according to claim 1, characterized in that the oligomeric maleimide or maleamic acid ranges from about 0.5 to about 5.0 phr. 6. Compound according to claim 1, characterized in that R is hydrogen, n is 0 and R² is an alkyl having 1 carbon atom and X is about 1 to about 146. 7. Compound according to claim 6, characterized in that the amount of oligomeric maleimide or maleamic acid ranges from about 0.5 to about 5.0 phr.